1. Field of the Invention
This invention relates generally to cardiac stimulator leads, and more particularly to a cardiac stimulator lead having an electrode selectively coated with an insulating material to define small conductive regions.
2. Description of the Related Art
Conventional cardiac stimulator systems consist of a cardiac stimulator and an elongated flexible cardiac lead that is connected proximally to a header structure on the cardiac stimulator and is implanted distally at one or more sites within the heart requiring cardiac stimulation or sensing. The cardiac stimulator is normally a pacemaker, a cardioverter/defibrillator, a sensing instrument, or some combination of these devices.
At the time of implantation, the distal end of a cardiac lead is inserted through an incision in the chest and manipulated by the physician to the site requiring electrical stimulation with the aid of a flexible stylet that is removed prior to closure. At the site requiring electrical stimulation, the distal end of the lead is anchored to the endocardium by an active mechanism, such as a screw-in electrode tip, or alternatively, by a passive mechanism, such as one or more radially spaced tines. The proximal end of the lead is then connected to the cardiac stimulator and the incision is closed.
Many implantable cardiac stimulators include a microprocessor or control circuit enclosed within a sealed housing or can. The circuit board controls the delivery of electric pulses to the lead and may perform various other functions. Power is supplied by an internal battery.
A conventional cardiac stimulator lead normally consists of an elongated, flexible, tubular, electrically insulating sleeve connected proximally to a connector that is adapted to couple to the header of a cardiac stimulator can and connected distally to a tubular tip electrode. One or more ring-type electrodes may be secured to the sleeve at various positions along the length of the sleeve. The proximal end of the lead sleeve is connected to the connector by application of various biocompatible adhesives to various portions of the connector and the sleeve. The tip electrode ordinarily consists of a tubular structure that has an increased diameter portion that forms an annular shoulder against which the distal end of the lead sleeve abuts. The exterior surface of the tubular structure is normally smooth as is the interior surface of the distal end of the lead sleeve. In multi-polar leads, one or more ring-type electrodes may be fitted over the sleeve.
To ensure that physical contact with the desired myocardial tissue is maintained after implantation, tip electrodes for most conventional leads are anchored to myocardial tissue by a fixation mechanism of one sort or another. In some leads, a corkscrew-like member projects from the tip electrode and penetrates the endocardium. In others, the electrode is fitted with one or more radially projecting tines that engage the trabeculae within the heart. Still others may employ both types of structures.
Most conventional tip electrodes serve at least two functions. First, tip electrodes provide a conducting member to convey electrical stimulation and sensing signals to and from myocardial tissue. Second, most tip electrodes provide structure to accommodate a fixation mechanism. Although conventional ring electrodes may be fitted with tines, most ring electrodes serve primarily as signal conductors.
The design of cardiac stimulation systems involves a balancing of a number of competing design considerations. Some of these include can size, lead tip dimensions and power consumption. Can miniaturization has been an important design goal since the first implantable pacemakers were introduced over thirty years ago. Smaller cans yield better post-operative comfort and cosmetic results for the patient. However, can miniaturization has required downsizing in storage batteries, which has, in turn, placed a premium on power consumption. Power consumption is of great importance because for a given level of power consumption, smaller batteries generally translate into shorter cardiac stimulator life spans and more frequent surgical procedures for the patient.
Some of the limitations associated with diminishing battery size have been offset by advances in cell chemistry. In addition, advances in pulse generation circuitry have dramatically increased the efficiency of power consumption. For example, many cardiac stimulators incorporate circuitry that automatically tailors pulse generation to the physiological demands of the patient.
However, despite advances in battery chemistry and circuitry, power consumption efficiency is still frequently limited by conventional lead electrode design. Most conventional lead electrodes operate as relatively low impedance, and thus, high current drawing devices. The low impedance levels are primarily a function of the relatively large conducting surface areas that these devices present to myocardial tissue. As noted above, the size of conventional lead electrodes is dictated in large part by mechanical considerations, such as the facilitation of fixation mechanisms. Furthermore, a certain degree of bluntness in a tip electrode is desirable to reduce the risk of myocardial perforation and micro-dislodgement, and to facilitate capture of the lead tip by post-implant developing fibrous tissue. Similarly, miniaturization of ring-type electrodes is generally limited by the size of the insulating lead sleeve and by the prevailing mechanical systems used to secure such ring-type electrodes to the lead sleeve.
As a result of these mechanical design considerations, current is often drawn by conventional low impedance electrodes at higher rates than necessary for appropriate stimulation. Some improvement in current drain may be realized by lowering the voltage output of the pulse generator. However, this technique is not possible in patients who require a threshold voltage for successful stimulation that is above the contemplated lowered output voltage. Thus, conventional lead electrode designs may represent an impediment to extended battery life.
In one conventional lead design, the distal end of the lead is provided with a distally projecting, small diameter circular electrode that has the potential to provide enhanced pacing impedance. However, this design may be prone to micro-dislodgment. Since the lead is provided with a single small conducting surface on the distal end of the lead, normal heart motion may cause the small conducting surface to momentarily lose contact with or micro-dislodge from myocardial tissue and disrupt the flow of pacing pulses.
The present invention is directed to overcoming or reducing the effects of one or more of the foregoing disadvantages.